1. Field of the Invention
The present invention relates to an optical unit. In particular, the present invention relates to an optical unit in which optical components are housed within a holder.
2. Description of the Related Art
Since the past, an imaging device using an image sensor element, such as a charge-coupled device (CCD) and a complementary metal oxide semiconductor (CMOS), mounted on a portable computer, a television phone, a portable phone, a digital camera, and the like, has been using a lens unit having a lens for forming an image of an object on a sensor surface of the image sensor element.
Within this type of lens unit, there is a lens unit that is assembled as follows. A lens is housed within a cylindrical holder while position adjustment (centering) of an optical axis of the lens is being performed. The housed lens is then pressed by a pressing component. Moreover, among lens units assembled in this way, there is a lens unit in which a tapered surface is formed on a flange section of the lens. The lens is housed within a holding section within the holder in a state in which the tapered surface is in contact with the holding section or a tapered surface of another lens (refer to, for example, Patent Literature 1).
Conventionally, when various lens units such as this that are mounted on imaging devices are mounted on a circuit board to which the lens unit is assembled to achieve modularization, the image sensor element is attached to a lens unit by a thermosetting resin and the like. A socket (an integrated circuit [IC] socket) is attached to the circuit board by reflow soldering. Subsequently, the image sensor element is fitted into the socket. A connector serving as the socket is described in Japanese Patent Laid-open Publication No. 2005-101778.
The socket is attached to the circuit board by reflow soldering and the lens unit is mounted on the circuit board by separate processes to prevent changes in optical performance caused by thermal expansion of the lens in the lens unit as a result of exposure to a high temperature environment during reflow soldering.
In other words, each component configuring the lens unit such as the lens, the holder, the pressing component, and a diaphragm, may be formed from materials with different linear expansion coefficients. If the lens unit is exposed to a high temperature at which reflow soldering is performed in an instance such as this, an amount of change in dimensions caused by thermal expansion of each component differs because of differences in the linear expansion coefficient of each component. As a result, the thermally-expanding lens presses against another component in the lens unit, such as the holder, another lens, or the diaphragm, and is affected by external stress from the other component. The lens that has been thermally expanded and affected by the external stress does not return to its original lens shape before thermal expansion, even when the reflow soldering is completed and the lens is cooled. As a result, changes occur in optical performance. To prevent a situation such as this, conventionally, the reflow soldering of the socket onto the circuit board and the mounting of the lens on the circuit board are required to be performed through separate processes
However, a number of processes increases when the mounting of the lens unit on the circuit board and the reflow soldering are performed by separate processes. Moreover, as a result of the socket being required, a number of component in a module increases.
As a method of mounting the lens unit on the circuit board without performing reflow soldering, there is a method in which, for example, after a flexible printed circuit (FPC) to which the image sensor element is attached to the lens unit by a thermosetting resin or the like, the FPC is fitted onto the circuit board. However, similar to when the socket is used, a method such as this causes an increase in the number of components because the FPC is required. Moreover, the FPC is expensive. Therefore, the method is not suitable for cost reduction.
Therefore, as described in Patent Literature 2, for example, a technology has been proposed that allows mounting of the lens unit on the circuit board to be performed in a same process as that for reflow soldering, while suppressing deterioration of the optical performance of the lens caused by the thermal expansion of the lens unit.
In other words, in Patent Literature 2, a lens unit is disclosed in which an elastic spacer presses a lens placed within a case, and an elastic ring surrounds an outer circumference of the lens, thereby holding the lens. In a lens unit such as this, a change in the dimensions of the lens during thermal expansion is absorbed by deformation of the elastic spacer and the elastic ring. Therefore, the external stress applied to the lens can be reduced to a certain extent.
Patent Literature 1: Japanese Patent Laid-open Publication No. 2007-163657
Patent Literature 2: Japanese Patent Laid-open Publication No. 2003-295024
However, even in the lens unit described in Patent Literature 2, external stress from the elastic spacer and the elastic ring may be placed on the lens when the lens and the like thermally expand. Therefore, the lens unit is not necessarily optimal for external stress reduction.
Moreover, in the lens unit described in Patent Literature 2, components separate from the lens, namely the elastic spacer and the elastic ring, are required. Therefore the increase in the number of components still occurs, similar to when the above-described socket and FPC are used.